/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.*;
import org.springframework.context.EnvironmentAware;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.env.EnvironmentCapable;
import org.springframework.core.io.Resource;
import org.springframework.core.io.ResourceEditor;
import org.springframework.core.io.ResourceLoader;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;
import org.springframework.util.StringUtils;
import org.springframework.web.context.support.ServletContextResourceLoader;
import org.springframework.web.context.support.StandardServletEnvironment;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Set;

/**
 * Simple extension of {@link javax.servlet.http.HttpServlet} which treats
 * its config parameters ({@code init-param} entries within the
 * {@code servlet} tag in {@code web.xml}) as bean properties.
 *
 * <p>A handy superclass for any type of servlet. Type conversion of config
 * parameters is automatic, with the corresponding setter method getting
 * invoked with the converted value. It is also possible for subclasses to
 * specify required properties. Parameters without matching bean property
 * setter will simply be ignored.
 *
 * <p>This servlet leaves request handling to subclasses, inheriting the default
 * behavior of HttpServlet ({@code doGet}, {@code doPost}, etc).
 *
 * <p>This generic servlet base class has no dependency on the Spring
 * {@link org.springframework.context.ApplicationContext} concept. Simple
 * servlets usually don't load their own context but rather access service
 * beans from the Spring root application context, accessible via the
 * filter's {@link #getServletContext() ServletContext} (see
 * {@link org.springframework.web.context.support.WebApplicationContextUtils}).
 *
 * <p>The {@link FrameworkServlet} class is a more specific servlet base
 * class which loads its own application context. FrameworkServlet serves
 * as direct base class of Spring's full-fledged {@link DispatcherServlet}.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see #addRequiredProperty
 * @see #initServletBean
 * @see #doGet
 * @see #doPost
 */
@SuppressWarnings("serial")
public abstract class HttpServletBean extends HttpServlet implements EnvironmentCapable, EnvironmentAware {

    /**
     * Logger available to subclasses.
     */
    protected final Log logger = LogFactory.getLog(getClass());
    private final Set<String> requiredProperties = new HashSet<>(4);
    @Nullable
    private ConfigurableEnvironment environment;

    /**
     * Subclasses can invoke this method to specify that this property
     * (which must match a JavaBean property they expose) is mandatory,
     * and must be supplied as a config parameter. This should be called
     * from the constructor of a subclass.
     * <p>This method is only relevant in case of traditional initialization
     * driven by a ServletConfig instance.
     *
     * @param property name of the required property
     */
    protected final void addRequiredProperty(String property) {
        this.requiredProperties.add(property);
    }

    /**
     * Return the {@link Environment} associated with this servlet.
     * <p>If none specified, a default environment will be initialized via
     * {@link #createEnvironment()}.
     */
    @Override
    public ConfigurableEnvironment getEnvironment() {
        if (this.environment == null) {
            this.environment = createEnvironment();
        }
        return this.environment;
    }

    /**
     * Set the {@code Environment} that this servlet runs in.
     * <p>Any environment set here overrides the {@link StandardServletEnvironment}
     * provided by default.
     *
     * @throws IllegalArgumentException if environment is not assignable to
     *                                  {@code ConfigurableEnvironment}
     */
    @Override
    public void setEnvironment(Environment environment) {
        Assert.isInstanceOf(ConfigurableEnvironment.class, environment, "ConfigurableEnvironment required");
        this.environment = (ConfigurableEnvironment) environment;
    }

    /**
     * Create and return a new {@link StandardServletEnvironment}.
     * <p>Subclasses may override this in order to configure the environment or
     * specialize the environment type returned.
     */
    protected ConfigurableEnvironment createEnvironment() {
        return new StandardServletEnvironment();
    }

    /**
     * Map config parameters onto bean properties of this servlet, and
     * invoke subclass initialization.
     *
     * Servlet 初始化
     * @throws ServletException if bean properties are invalid (or required
     *                          properties are missing), or if subclass initialization fails.
     */
    @Override
    public final void init() throws ServletException {

        // Set bean properties from init parameters.
        /**
         * 获取配置信息
         * {@code servlet-> init-param}
         */
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                // bean包装给spring管理
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
                bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
                // 空实现 具体子类做
                initBeanWrapper(bw);
                // 设置属性
                bw.setPropertyValues(pvs, true);
            }
            catch (BeansException ex) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();
    }

    /**
     * Initialize the BeanWrapper for this HttpServletBean,
     * possibly with custom editors.
     * <p>This default implementation is empty.
     *
     * @param bw the BeanWrapper to initialize
     * @throws BeansException if thrown by BeanWrapper methods
     * @see org.springframework.beans.BeanWrapper#registerCustomEditor
     */
    protected void initBeanWrapper(BeanWrapper bw) throws BeansException {
    }

    /**
     * Subclasses may override this to perform custom initialization.
     * All bean properties of this servlet will have been set before this
     * method is invoked.
     * <p>This default implementation is empty.
     *
     * @throws ServletException if subclass initialization fails
     */
    protected void initServletBean() throws ServletException {
    }

    /**
     * Overridden method that simply returns {@code null} when no
     * ServletConfig set yet.
     *
     * @see #getServletConfig()
     */
    @Override
    @Nullable
    public String getServletName() {
        return (getServletConfig() != null ? getServletConfig().getServletName() : null);
    }


    /**
     * PropertyValues implementation created from ServletConfig init parameters.
     */
    private static class ServletConfigPropertyValues extends MutablePropertyValues {

        /**
         * Create new ServletConfigPropertyValues.
         *
         * @param config             the ServletConfig we'll use to take PropertyValues from
         * @param requiredProperties set of property names we need, where
         *                           we can't accept default values
         * @throws ServletException if any required properties are missing
         */
        public ServletConfigPropertyValues(ServletConfig config, Set<String> requiredProperties)
                throws ServletException {

            Set<String> missingProps = (!CollectionUtils.isEmpty(requiredProperties) ?
                    new HashSet<>(requiredProperties) : null);

            Enumeration<String> paramNames = config.getInitParameterNames();
            while (paramNames.hasMoreElements()) {
                String property = paramNames.nextElement();
                Object value = config.getInitParameter(property);
                addPropertyValue(new PropertyValue(property, value));
                if (missingProps != null) {
                    missingProps.remove(property);
                }
            }

            // Fail if we are still missing properties.
            if (!CollectionUtils.isEmpty(missingProps)) {
                throw new ServletException(
                        "Initialization from ServletConfig for servlet '" + config.getServletName() +
                                "' failed; the following required properties were missing: " +
                                StringUtils.collectionToDelimitedString(missingProps, ", "));
            }
        }
    }

}
